Bioflotation treatment of oil-containing waste water



Aug. 9, 1960 R. J. AUSTIN ETAL 2,948,577

BToELoTATToN TREATMENT 0E ETL-CONTAINTNG wAsTE WATER A Filed Aug. v29,1956 ATT ORNE Y v INVENTORS.- Rober! J. Austin James E vErl/Isch Y,

` 6. M /aff w .al

United States Patent Q,

BIOFLOTATION TREATMENT '0F OIL-'CONTAIN- ING WASTE WATER Robert J.Austin, Crete, Ill., and James Grafsch, Hammond, and Russell C. Mallatt,Crown Point, Ind., assignors toStandard Oil Company, Chicago, Ill., acorporation ot? Indiana Filed Aug. 29, 1956, Ser. No. 606,919

2 Claims. (Cl. Z310-44) This invention relates to .an improved system-for treating Voil-containing waste water by an improved bioilotationtechnique and it pertains more particularly to the treatment of,petroleum refinery waste water so that it maybe discharged into lakesor streams without undueV contamination thereof.

The Aproblem of wastejdisposal in the petroleum Vindustry has becomevery acute but, in spite of -intensive research by those most highlyskilled in the art, no simple inexpensive .technique has heretofore beendeveloped which enables the production of .an etiluent water which .is.su-ffiiciently free from oil and obnoxious chemicals, taste, odor andbacteria to meet the strict requirements of regulatory bodies. An objectof this invention is to lprovide such .a technique. Another lobject isto provide an improved method and means for effecting aeration ofpetroleum reii-nery waste water. A further object is to attain thecombined advantages of prior activated sludge and flotation processeswhile Vavoiding their disadvantages. Other objects will be ,apparent asthe detailed description of the invention proceeds.

Waste disposal in the petroleum industry is discussed ina series ofpapers published in Industrial and Engineering Chemistry, volume 46, No.2 (February 1954.), at pages 283 to 333. Petroleum waste waters fromconventional American Petroleum Institute (A.P.I..) separators containorganic matter and/or organisms which require biochemical oxidation andthe biochemical Voxygen demand (B.O.D.) of such waste may be of theorder of about 30 to 200 or more parts per million (,-p-.p.m.,); -inlakes and streams the B.O.D. is usually .less than 1 p pim. and eflluent-water discharged thereto should have a B.O.D. of less than ppm. andpreferably less than about 6 p pm. Such secondary wastes contain oil indissolved 4or solutized form to the extent of about 100 to 200 ppm.; itis desirable that the oil content be reduced vto not more than about 30p.p.m. and to as great an extent as possible. The phenolics content ofsuch waste may range .from about f1 to 15 ppm. or more and it isdesirable that the phenolics be reduced to not ksubstantially more thanabout .3 ppm. and preferably to la much lower ligure. The threshold odornumber (T.O.N.) of the secondary waste may be of the order-of 20,000 to50,000 or more and it should be reduced to `not more than about 6,000 orlower. The extent of required purification will, of course, -depend uponeach particular situation. lThevgenera-l appearance ofthe waste water`is also an important factor, it being desirable that the dark coloredand almost black waste with an oily caste be converted into a relativelyclear water of lighter and brighter color.

Our invention 'is ,preferably applied to the. petroleum refinery 'wastedischarged from the usual A.P.I. separator. We Lfirst 'adjust theacidity of this waste water to a level in the pH range of about 61"/2 to81/2, esg. about `7 and, it necessary, adjust its temperature to alrange of about 40 to l0`0 F., preferably about 90 F. 'If the "wasteywater'contains' excessive amounts of sultde's, particularly HES, .itfis desirable to 'strip 'out HZS by passing air upwardly through thewater Afrom distributors or diifusers at the base "of a preliminarystripping zone; in most cases, rhoweve'r', Athis stripping 'step' is notrequired 'since our beaters niay aceomplish 'the required stripping. ThepH adjusted 'stream at Athe Idefined temperature is then preferablypassed through Aa preliminary settling zone, wherein it is he'ld y(for'about .2 to '4 h'ours or 'more to enable 'removal by sedimentation andnotation of lsub- Astances which can thus .be removed, 'therebyminimizing the load on the subsequent 'bioota'tion treatment. The bottom`sedirrierit accumulates so slowly that it may be pllbdlally' 'I'lUVfiltVlS f 'b'O'U a Yel 1 mre but any oil or 'sonni should be'skimtnedtroni -the top of the water leaving the preliminary settler before it issubjected to our bioliotatioii treatment. Y

In our bioiio'tation treatment 'the preseftled -waste water at 'thedefined aeidity and 4temperature `is passed in `series' through a`plurality of Yaerobic zones with holding times in each zone Ain therange of about ..3 to 3 hours, preferably about l liourj, Sand with anaverage 'flow rate through said zones in the range of about .1 to l0'feet per minute, preferably Vabout f5 to 5 or 41 'to 2 feet ,perminute. Beaters areins'talled at the inlet surface of each aerobic zonefor 'beating or triturat'ing air -into the water whereby a myriad offine bubbles are formed, dispersed and impelle'd into the water so thatit assumes a somewhat milky appearance.

The beating may `be eifected by means of so-called brush aerationalthough lfor our ,purpose the lbeater or brush elements should have awidth which is not substantially greater than ltheir. thickness since wewant to minimize a paddle wheel eifect which would cause'ithe uppersurface of the water to `flow more rapidly than Ithe main body vthereofand thereby set up a morel or less `turbulent circulation. We haveobtained -outstanding'lfy superior results Vv'vit'h beater ele-mentsfabricated .from expanded metal having sharp edges and angular openingsof about 172 to l inch, preferably about 3A finch, the elements inl thiscase being the portions of the metal surrounding the openings and therelatively large openings minimizing the paddle wheel effect. nstead ofexpanded metal 'we may use hardware cloth, cyclone fencing, or otherrigid foraminous material-having square or diamond shaped openingsprovided that the openings are l/z to l inch, but these alternatives arenot the full equivalent ofexpanded lmeta-l. u A' `-Our lbeaters A may befabricated by radially .mounting sheets or blades of expandedmetal on acentral tubular shaft, the blades extending joutwardly about r4 to .20inches from the rotating shaft and being 'spaced at about 30" intervalsand .suitably braced at spaced' horizontal intervals by welding discs,disc sectors., Jbars or .rings thereto to provide strength and uniformweight balance. The beaters are rotated' by electric motors or any otherdrive means, preferably at' about v5'() to '250, e.g.` about 1'2'0r.p.rn.; with an "18 inch outside diameter beater the peripheral beaterspeed may be Vof the order of about '2 to 20 feet per second which isour preferred range. The immersion depth of `the beatersV is usuallyabout 1 to 6 inches, preferably about l2- to 3 inches, the bearingsupports for 'the beater shafts being preferably vertically adjustableitl 'older' to obtain 'the desired level .of immersion. u A- vDownwardl'y` inclined bailles are' mounted vadjacent the downstream sideof the beaters, 'preferably at an 'angle of about 20 "to 5'0', thenpp'er'edge ofthe baffle being' above or at the approximate level of theupper "part of 'thebrush and the lower endr ofthe beine extending about2 inches to a foot or more beneath thesu'lfa'ce of the'tfatefr.` #The'pnrposeo'f tb-e battle istof `irrlprtwe aeration eiiieieney (notePasveers Researchx 'on Activated' Sludge, Sewage and Industrial Wastes,November 1953, pages 1253-1258, and December 1953, pages 1397-1404) butin our system it serves an added function of downwardly directing theliquid containing entrained air bubbles and thus substantially limitingany turbulence to the inlet end of each aerobic zone so that the liquidin the downstream portion of each zone may remain relatively quiescent.The combined action of the beater and the baffle causes the gas bubblesto be directed to the'bottom of the tank so that oxidation andsubsequentiiotation is effected throughout substantially the entire massof theliquid.

After the initial biological growth Vperiod which may require severalweeks,- particles which may be called sludge but are better describeda's slime' appear and grow, a relatively'small part tending tobe'internally recycled at the inlet end of each aerobic zone and alarger part accumulatingin bulk in thel downstream lportion of the zone.This biological Vslime functions in a manner I` analogous to 'sludge inVaccelerating both chemical and biological oxidation so that most of theoxygen content of the entrained bubbles is effectively utilized. Theslime particles apparently provide nuclei or absorbtive floc forcoalescing oil and entrained organic matter and at the same time theytend to occlude the nitrogen and unused portion of the oxygen, theoccluded gas Atending to make the particles suiciently buoyant so thatthey iioat to the surface of the liquid in the downstream quiescentsection of the aerobic zones. We have found that for best results it isessential to provide a holding period of at least about .3 hour in orderto effectively utilize the entrained oxygen and that it is alsoessential to remove the slime which is oated to the surface because ifthis slime becomes vdeaerated it will sink back Iinto the liquid and/or.

While our invention may be useful in the treatment of any oil-containingwaste water whether the oil be of animal, mineral or vegetable origin,it must be recognized that diiierent types of waste waters presentdifferent problems and our invention is particularly applicable to thetreatment of petroleum`refinery'waste water discharged from A.P.I.separators. Such waste, water may be charged,ffor example, through line10 to neutralizer vesL sel 11 provided with a motor driven stirrer 12such as a Lightnin mixer and a neutralizing agent, such as aqueoussodium hydroxide, is introduced into the mixing vessel through line 13at a rate to bring the acidity of the waste water to a level in the pHrange of 61/2 to 8%,

' e.g. about7. 1A pH Vdetermining instrument 14 may be mechanisms.

connected to a suitable recorder 14a and the rate of causticintroduction may -be manually or automatically controlled in accordancewith the indicated pH by known If the waste water is on the alkalinerather than on the acid side, a suitable acid such as sulfuric acid isintroduced through line 13 instead of aqueous sodium hydroxide.

The pH adjusted water is then introduced by line 15 to the presettlingzone 16 and the temperature of the water accumulate on ,the bottom ofthe zone, thereby decreasing 'the effectiveness of the treatment. Bylimiting the holding period to approximately 1 hour, we can electivelyutilize the oxygen content and at the same time `recover substantiallyall of the resulting oily slime from the sury face of the water by askimmer. The technique thus dened enables a large part of the impuritiesto be removed without necessity of biological or chemical oxidation thusenormously minimizing the overall oxygen requirements of the treatingsystem.

The number of aerobic zones employed will depend on Athe nature of thewaste water and the requirements for nal effluent purity. We prefer toemploy at least three zones in series and in one commercial designvesuch zones are utilized. Each zonev is preferably from about 15 to300, preferably about 50 to 200, feet in length with "l f a'water depthof about 3 to 10 feet and with a width to provide the beater lengthnecessary for incorporating the 'required amount of oxygen and otationgas in said zone, '1 foot of beater length usually being adequate toincorporate about 6 to 10 pounds per day of oxygen and 24 to 40 poundsof inert gases into the water. The inlet to each zone is preferably fromnear the bottom of the preceeding zone after slime is skimmed therefrom.

4 Total beater length which may be divided up into a plurality of stagesis about 1 foot for each 5 to 100 gallons per minute of waste water tobe treated dependingupon the extent of treatment required.

The invention will be more clearly understood from the followingdetailed description of a specific example thereof read in4 conjunctionwith the accompanying drawings Ywhich form a part of this specificationand in which:

Figure l is a schematic vertical section of our treating system, i

Figure 2 is a schematic top view thereof,

Figure 3 is a schematic vertical section of a beater,

Figure 4 is a transverse section taken along the lines 4-4 of Figure 3,

Figure 5 is a schematic plan view of an alternate biokflotation treatingsystem, and

Figure 6 is a detailed section illustrating an adjustable ,mounting fof.the beater @sembla beater) being about 10 feet.

in the presettling zone should be in the range of about 40 to 100 F.,preferably about 90 F. since that temperaf ture favors the biologicalgrowth in the subsequent bioflotation step. Where the waste water is athigher temperatures, it is preferably cooled by introducing cold waterfrom a lagoon or other source through line 17. Colder waste water may beheated to the desired temperature by means of steam coils or in anyother conventional manner.

When the waste water contains large amounts of suldes, particulary H2S,it may be desirable to pass it through a hydrogen sulfide stripper 18before it enters presettler 16. Any conventional type of stripper may beemployed such, for example, as a tank provided with suitable batiies 19,an air inlet 29 and a suitable air distributor such as diffuser plates21.

Presettler `16 may be a cement lined channel with side walls about 6 tol0 feet high to accommodate a usual water depth of about 4 to 5 feet,the channel being about v12 feet wide and the settling zone being about100 feet long. With the waste water inlet of about 1 million gallons perday, the presettler will thus provide a holding time of about 1 hour anda flow rate of about 1.6 f eet per minute. A skimmer 22 is providedadjacent the downstream side of the presettler for removing any oil orother matter which rises to the surface in the presettler. Batlle 22extends from above the surface of the water to the lower part of thetank but is spaced from the bottom thereof. Solids 23 may accumulate atthe base of the presettler but at a very low rate of only a few inchesper year so that usually there is no necessity for continuous removalthereof.

The skimmed water flows from beneath baie 22 over the Weir or darn 24into the first biootation zone 25 which in this example is the same sizeand shape as the presettler. Beater 26 is mounted to rotatecounterclockwise with the beater elements extending about 2 to 3 inchesbelow the surface of the water. The beater of this example is shown moreclearly in Figures 3 and 4 and it consists of a transverse horizontaltubular shaft 27 mounted in vertically adjustable bearings 28 andprovided with a driving means 29. In this example the tubular shaft isabout 4 inches in diameter and l2 outwardly extending (radial,tangential or the like) strips 30 of expanded metal are welded orotherwise secured to the shaft at 30. intervals, the Width of each stripbeing 7 inches and the horizontal length of the strips (i.e. of the Foradditional firmness supports in the form of annular rings 31 or discsectors are welded to the expanded metal strips or blades at spacedintervals, the beater being carefully weight-baL .ancedhto preventvibration at high speeds of rotation.

changera The expanded metal strips .are preferably fabricated fromcorrosion resistant steel and are characterized by :relativelysharpedges land diamond shaped openings vof about 3/4 inch. The overallbeater .is thus 'about -18 inches in diameter and when rotated at about1.20 -r.p.m. Iit will provide a peripheral speed `of about feet Apersecond. A method of .mounting the beater .bearings .-for verticaladjustment is illustrated in Figure 6, I-beams 32 being verticallysecured `.to concrete piers fand an L-:shaped bearing support 33 l.beingadjustable upwardly or downwardly by ycable 34 for .raising :andlowering bearing support .35. Batlle 36 is likewise supported .on thelL- shaped member 33, .the upward end f'of 'the bame :being above .thebeater 4and A[the lower ilend extending downwardly at an .angle of4about 40 a :few Lincheshelosv the lower level of the beater (about l6inches below .the surface of the Water). -The baille Amay .beapproximately 3 inches from the .periphery of the beater .at its closestpoint.

When beater brush 2.6 is rapidly .rotated as hereinabove described airis dispersed into the water in the .form of myriads of minute vbubblesand the water containing such bubbles is impelled downwardly toward thebottom of the tank at the inlet vend of vzone .25, partly because fof.the impelling action of the expanded metal beater blades and partlybecause of the dellecting action of baille .36. For best results theentrained air bubbles should be extremely small in size but large innumber 4and should give the water a somewhat milky appearance. Thesebubbles become affixed to slime particles previously formed andinternally circulated at the inlet end of the zone but most of theparticles flow downstream and are gradually buoyed or floated to the.surface while `the bacterial oxidation is taking place. Oil and otherorganic -contaminants apparently are coalesced on the slime orbiologically induced lloc particles and likewise buoyed upwardly by theunconsurned portion of the aerating gas. It is important that theholding time in zone vbe at least about .3 hour in order to effectivelyutilize flow rate of about 1.6 feet per minute kprovides the requiredquiescence for the flotation. The oily slime is removed by skimmer 37and the water is withdrawn from the bottom of the tank under bafe 37 sothat any settling solids are carried over to the next zone. It isdesirable to prevent accumulation of solids at the bottom of theaeration zones.

Another Weir or dam 38 separates the ilrst bioilotation zone 25 from thesecond biollotation zone 39 which is provided with a beater 26a, bafe36a, skimmer 37a and baille 37a which function in the same manner asbeater 26, baille 36, skimmer 37 and baille 37', respectively. In thesecond biollotation zone there is a further reduction of impurities inthe waste water but usually a lesser reduction than accomplished in thellrst zone. The skimmed water from the second bioilotation zone 39 llowsover weir or darn 40 into the third biootation zone 41 which is providedwith beater 26h, baille 3617, skimmer 37b and baille 37b, the size,arrangement and operation of this zone being as heretofore described inconnection with the irst zone. The final eilluent which leaves thesystem through line 42 is of such purity that it may be introduced intoa sufhciently large lake or stream Without further purillcation. In somesituations it may be necessary to provide a number of additionalbioilotation stages and/or to discharge the nal eilluent into a pond oflarge surface area so that the nal oxidation may be thus effected inaccordance with -known technique.

In Figure 5 we have shown an alternate flow plan wherein the petroleumwaste water after pH adjustment is introduced 'into .Qa :settling 'zone'43 :provided 'with .skimmer V44 and baille 44'., then passed Aoverwe'ir r45 Iunder beater 46 and baille 47 'into the rst b'iofl'otationfzon'e 48 and thence in a similar manner through biootaton zones 49, 50,51 .and .52, the treated Water being lwithdrawn through fline :53. .Inthis Isystem `the required length oflzones Lis attained byback and forthflow `through adjacent channels and this or similar :systems vmay beemployed depending upon the space available .for -the treating unit. 5inthis system as 'well sas in the `previously described s-ystem skimmingsfrom :all zones are removed #either continuously or lat frequen'tintervals. Ethesefskirnmings are .largely "water butsmay contain about dto percent of foil, .2 to .r3 :percent ash and about f5 to 5 percent "ofbiological fslime. The skimmings .may be Jdewatered and dried by `anyknown mechanical and/ or 'chemical technique and then burned or may 'bedis posed -of .in any other rblcnown .'manner.

As .an example of results obtainable by our biodlotation process, a"test was zmade in adjacent channels by llowi-ng :equivalent portions.of waste water through 3 zones each having a detentiontime of .1% hoursand `each being geo'- -metrically the same as vthe other. .In'theso-'called ygravity channel each "of the zones 'was simply employed as1a .settling zone while in "the lsoa-called aerated `sidethe first ofthe zones was employed fas asettler and the next two zones were providedwith beaters and 'adjacent baffles substantially as hereinabovedescribed. Initially there 'was 'no substantial difference .in thepurification accom- -plished inthe gravity .side :and the vaerated siderespectively ybut after a few weeks operation the biological life-appeared in :the aerated side of the tank `and after biological slimehad accumulated, the following data were obtained:

First Second f 'lhlrd Characteristics Feed 1 1% -l 1% Hour Hour HourZone Zone Zone 'B.O.D.,p.p.m.:

Gravity Side 62 51 48 46 Aerated Side 62 51 12 6 `r1.O.'N.:

Gravity Side-.- v19, 000 a6, 000 16, 000 13, U00 Aerated Slde. ..u19,000 16,000 10,000 6, 000 O11, p.p.-m.:

Gravity-Side 1'18 71 76 53 Aerated Side 11'8 71 38 30 Phenolles, p.p.m.

Gravity Side. 1. 34 81 1.04 1. 28 Aerat-ed Side 1. 34 81 36 .31

effective purification at about one-third to one-fourth of theinvestment cost that would be required for obtaining similar results byother known methods. The ellluent is relatively clear and bright.

It will be noted that the bootation process hereinabove described doesnot require theuse of added otation materials. On the contrary, itemploys the buoyed slime itself to serve this function. This is aradical departure from prior flotation techniques and it likewise isdistinctly dilerent from activated sludge processes herefore employed;it attains the advantages but avoids the disadvantages of these separateand Idistinct prior processes. It should be understood, however, that insome cases it may be advantageous to add a known type of oc formingchemical agent such as alum, ferrie sulfate, activated silica, etc. orother known type of surface active material provided that the addedmaterial does not injpair the eectiveness of the biological action andprovided thatl it is actually beneficial in -increasing the flotation ofimpurities to be eliminated. The buoyancy effect of residual nitrogen orunused aeration gas for improving the effectiveness of otation may beapplied to improve the etfectiveness of other gravity settling systemsmuch in the manner in which such buoyancy is utilized in ourbioflotation process.

While we have described a particular example of our invention inconsiderable detail, it should be understood that alternativearrangements, steps and conditions will be apparent from the foregoingdescription to those skilled in the art. Bafes 22', 37', 37'a, 37b and`44 may depend from or be integrally associated with skimmers 22, 37,37a, 37b and 44 respectively. Advantageous results may be obtained by`the use Vof previously known types of brush aerators even though suchbrush aerators are not necessarily the equivalent of`our beater,particularly when the so-called brush aerators are provided with bladeswhich unduly augment the paddle wheel effect. Brush elements may, forexample, be relatively rigid rods or wires of metal or plastic (such asnylon or other known materials) or natural fibers provided that they areeEective in producing and impelling into the body of the water themyriad of air bubbles of desired smallness. The outside diameter of thebeaters may range from about inches to 5 feet or more.

We claim:

1. The method of treating oil-refinery waste water containing oil indispersed form which comprises adjusting its acidity to a level in thepH range of about 61/2 to -81/2,passng said oil renery waste water at atemperature in the rangeY of about' 40 to 100 in series through aplurality of aerobic Zones with holding times in each ,zone in the rangeof about V.3 to 3 hours, triturating air into the water at the uppersurface of the water in a horizontal area extending across the upperinlet end of each of said zones to form, disperse rand impel downwardlytherein a myriad of ne air bubbles, restricting any increased surfaceHow rate and turbulence caused by the beating step to the inlet end ofeach zone whereby the water downstream therefrom is relatively quiescentso that growth of bacterial slime is promoted, oil and organic mattercoalesce thereon and oil-slime particles rare buoyed to the surface byresidual components of entrapped air bubbles, skimming oatingaccumulated oily slime from the upper part of each zone so thatsubstantially scum-free water is discharged therefrom and employing anaverage yflow rate throughsaid zones in the rangeof about .1 to 10 feetper minute.

2. A method of treating oil refinery waste water containing organicmatter and oil in dispersed form which comprises passing said oil reneryWaste water at a temperature' of 40 to 100 F. through an aerobic zonehaving an 4inlet and an outlet; triturating air into said waste water atthe upper surface of said waste water and at the inlet end of saidaerobic zone to form, disperse, and il'pell downwardly therein a myriadof tinely divided and highly dispersed air particles; limitingturbulence to said inlet end of said aerobic zone so that said wastewater in the downstream portion of said aerobic zone remains quiescentso that oxidation and subsequent flotation is effected throughout saidwaste water and whereby slime particles coalesce said oil and saidorganic matter and whereby the combined oil and slime occlude said airand whereby the oil, slime and air are buoyed to the surface of saidwaste water; skimming said oil, slime and air from the surface of saidwaste water, and subsequently withdrawing water which is reduced in oiland organic matter content from said outlet of said aerobic zone.

References Cited in the tile of this patent UNITED STATES PATENTS729,297 Edson May 26, 1903 1,525,394 Jolicoeur Feb. 3, 1925 1,843,647Bramley-Moore Feb. 2, 1932 1,893,623 Imhoi Jan. 10, 1933 2,058,785Goudey et al.' Oct. 27, 1936 2,094,909 Bailey et al. Oct. 5, 19372,129,267 Fischer Sept. 6, 1938 2,242,639 Barton May 20, 1941 2,281,609Walter May 5, 1942 2,322,017 Hartman lune 15, 1943 2,370,974 LangdonMar. 6, 1945 2,684,941 Pasveer July 27, 1954 2,766,203 Brown et al. Oct.9, 1956 2,798,042 Cox July 2, 1957 FOREIGN PATENTS 363,267 Germany Nov.6, 1922

1. THE METHOD OF TREATING OIL-REFINERY WASTE WATER CONTAINING OIL INDISPERSED FORM WHICH COMPRISES ADJUSTING ITS ACIDITY TO A LEVEL IN THEPH RANGE OF ABOUT 61/2 TO 81/2, PASSING SAID OIL REFINERY WASTE WATER ATA TEMPERATURE IN THE RANGE OF ABOUT 40 TO 100*F. IN SERIES THROUGH APLURALITY OF AEROBIC ZONES WITH HOLDING TIMES IN EACH ZONE IN THE RANGEOF ABOUT .3 TO 3 HOURS, TRITURATING AIR INTO THE WATER AT THE UPPERSURFACE OF THE WATER IN A HORIZONTAL AREA EXTENDING ACROSS THE UPPERINLET END OF EACH OF SAID ZONES TO FORM, DISPERSE AND IMPEL DOWNWARDLYTHEREIN A MYRIAD OF FINE AIR BUBBLES, RESTRICTING ANY INCREASED SURFACEFLOW RATE AND TURBULENCE CAUSED BY THE BEATING STEP TO THE INLET END OFEACH ZONE WHEREBY THE WATER DOWNSTREAM THEREFROM RELATIVELY QUIESCENT SOTHAT GROWTH OF BACTERIAL SLIME IS PROMOTED, OIL AND ORGANIC MATTERCOALESCE THEREON AND OIL-SLIME PARTICLES ARE BUOYED TO THE SURFACE BYRESIDUAL COMPONENTS OF ENTRAPPED AIR BUBBLES, SKIMMING FLOATINGACCUMULATED OILY SLIME FROM THE UPPER PART OF EACH ZONE SO THATSUNSTANTIALLY SCUM-FREE WATER IS DISCHARGED THEREFROM AND EMPLOYING ANAVERAGE FLOW RATE THROUGH SAID ZONES IN THE RANGE OF ABOUT .1 TO 10 FEETPER MINUTE.